Coffee beans are harvested in a form called green beans and are not useful for brewing coffee until they are roasted. Green beans are roasted by subjecting them to heat at a temperature, and for a time, to produce physical and chemical changes. The physical changes mostly involve driving off water and volatile organic compounds. Chemical changes involve limited pyrolysis which chars or carbonizes the organic compounds in the beans, and the pyrolysis creates more volatile materials, which are driven off, and slightly carbonized residues. The taste of coffee depends to a large extent on the character and quality of the green beans, but it is strongly influenced by the roasting process. Human taste is a very discriminating sense that is capable of distinguishing small differences in taste resulting from small differences in the roasting procedure. These small differences frequently make the difference between good coffee and bad coffee.
Green coffee beans are conventionally roasted by radiation caused by direct exposure to flame in a roasting oven. The beans are maintained in a rotating drum and tumbled as they are exposed to flame and hot gas. This process creates three major problems. One problem is that each bean is not subjected to the same roasting conditions as each other bean because they occupy different positions along the axis of the roasting oven. A second problem is that the window of opportunity to stop the roasting process when the maximum benefit from roasting has been achieved is very small. The roasting process does not stop abruptly when the flame is turned off. The residual heat in the beans causes the roasting process to continue until they are removed from the oven and cooled. The higher the temperature of the beans the more critical it is to stop roasting at exactly the right moment. Finally, the flame and hot gas used in conventional roasting cause deposits of undesirable combustion residue on the roasted beans.